Support 'discard sectors' operation.

We can benefit from knowing that the file system no longer cares about
the contents of certain sectors, by throwing them away immediately and
then never having to garbage collect them, and using the extra free
space to make our operations more efficient. Do so.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>